1. Field of the Invention
The present invention relates to an image processing method, an image processing apparatus, and an image forming apparatus. More specifically, the invention relates to a binarization processing technique for generating a halftone image by forming a halftone dot having a predetermined size corresponding to an intensity of an input image signal, the halftone dot which is used to record a gradation image on an image recording medium in a printing technique such as an electrophotographic method and an inkjet method.
2. Description of the Related Art
As one of the techniques for generating a gradation image using binary data, there has been known a method for generating a gradation image by using the pulse-surface-area modulation, that is, so called binarization processing method (particularly referred to as a halftone processing method)) in which colored dots called halftone dots (set of individual halftone output dots), each having a predetermined size corresponding to an intensity of an input image signal, are formed to thereby reproduce the density of a halftone image by the size of each colored dot.
For example, a color printed matter is obtained by printing respective inks, each having one of four colors composed of yellow (Y), magenta (M), cyan (C), and black (K) colors, on a recording medium (printing paper) with the inks superposed on one another subsequently, using four printing plates for the inks. On the printing plates are recorded halftone plate images in which gradation of continuous-tone images of a color manuscript is reproduced with a set of microscopic halftone dots.
For example, when generating a halftone plate image in a printing technique using an electrophotographic method, a comparator compares multilevel-image signals (multilevel data) representing the gradation of an image of a color document with predetermined screen pattern data (threshold data at a predetermined coordinate in a threshold-value matrix; hereinafter may be referred to as a threshold value), to generate binarized recording signals.
Further, the halftone plate images are exposed on an image formation member (for example, a photoconductor drum) by controlling on/off of a light beam for exposure according to halftone dot signals, using the binarized recording signals as on/off signals (halftone dot signals) for each record pixel. Then, toner (powder) is sprayed onto the image formation member to visualize an image on the image formation member (a latent image) as a toner image. Thereafter, the toner image is transferred and fused onto the image recording medium to form an image having halftone dots having a size corresponding to the density of the image.
Here, when the halftone dots are used in the electrophotographic method, in general, one or two grains (1.5 grains in average) of toner are piled up, reaching a height in a range of ten and several μm before a toner image is fused. Since the height of the piled-up toner is in many cases determined by an amount of toner required for the maximum density of the image, it may be an excessive amount of toner for halftone reproduction. In particular, since the size of a halftone dot is small in highlight tone area (low density region), there are high possibilities that this problem occurs.
For color reproduction, a thin halftone-dot toner image is needed in the transfer process of toner because deterioration of image quality during the transfer process increases as the thickness of a halftone-dot toner image is larger. In addition, for a multi-transfer for the color reproduction, more attention should be paid to the deterioration of image quality. However, it is difficult that the amount of toner needed for the maximum density is compatible with the amount of toner appropriate for the halftone dot reproduction.
Further, an unfused toner image having a thickness in the range of ten and several μm is crushed into a fused toner image having a thickness of several μm after it is fused. When the toner fused on paper absorbs light, density reproduction by the toner occurs. In order to enhance the light absorption efficiency, it is required to efficiently expose a coloring material containing a thin toner layer to light. However, as described above, in the halftone-dot structure for the halftone reproduction, the toner layer may become excessively thick in many cases, and therefore, the toner which makes a low contribution to light absorption exists on the paper.
On the other hand, in a field of a printing technique, such as an inkjet method, using ink as a coloring material, patent documents 1 to 5 disclose a technique of controlling the amount of ink adhesion for forming halftone dots for the purpose of adjusting the thickness of the halftone dots called a dot gain or transferability of ink (coloring material).
[Patent Document 1] WO 00/72580
[Patent Document 2] U.S. Pat. No. 6,532,082
[Patent Document 3] JP Sho.62-216748 A
[Patent Document 4] JP Hei. 3-053951 A
[Patent Document 5] JP Hei. 10-13685 A
For example, for the purpose of reducing the dot gain of a stochastic screen (stochastic printing), mechanism disclosed in the patent document 1 is a technique for appropriately reducing the density of a binarized image by further stochastically thinning out an image binarized with the stochastic screen.
In addition, mechanism disclosed in the patent document 2 is a technique for appropriately reducing the density of a binarized image by stochastically thinning out the image binarized by a normal halftone process, premised on halftone dots of clustered dots.
More specifically, in the mechanism disclosed in the patent document 1, with respect to the stochastic screen called an FM screen; and in the mechanism disclosed in the patent document 2, with respect to a regular halftone screen called an AM screen, the dot gain and the amount of ink are adjusted by non-periodically thinning out some of the halftone dots. That is, halftone dots and gap dots are asynchronously generated.
In particular, in the mechanism disclosed in the patent document 2, a normal halftone-dot image and an image representing a gap dot for asynchronously thinning out halftone dots are prepared, and the two images are combined to thereby generate a halftone-dot image having gap dots.
However, in the techniques disclosed in the patent documents 1 to 5, it is necessary to redesign parameters for the binarization process whenever the gap size or the frequency of gap generation changes, and it is difficult to cope with the change.
Further, when some of the halftone dots are thinned out, the density of an output image becomes lower than the density to be originally output, that is, an error occurs in the density of the output image.
Furthermore, in the mechanisms disclosed in the patent documents 1 to 5, there is a possibility that an isolated dot is generated, that tone jump occurs or that graininess deteriorates. For example, in the FM screen such as the mechanisms disclosed in the patent document 1, since the density of the image is reproduced with a minute density of dots, which are invisible (30 μm or less), some of the integrated (clustered) minute halftone dots may be thinned out and areas of colored pixels may be too small to reproduce dots stably.
On the other hand, in the AM screen such as the mechanism disclosed in the patent document 2, when some of the halftone dots are non-periodically thinned out, there may occur a case where some of the halftone dots are thinned out inside the halftone dots and a case where some of the halftone dots are thinned out outside the halftone dots. Accordingly, there may occur a phenomenon that the crush of some of the halftone dots is different from the crush of other halftone dots, which may result in image noises. In addition, a coloring material in a halftone dot portion may be made thin when many pixels are thinned out inside the halftone dots. However, when many pixels are thinned out outside the halftone dots, since the size reduction of the halftone dots is significant but an operation of thinning out the coloring material in the halftone dot portion is weakened, an effect of making the halftone dots uniformly thin can not be expected. In particular, since the size of the halftone dots becomes small in highlight tone area (low density area), there are high possibilities that the above-mentioned problems occur.
In addition, even in the mechanism disclosed in the patent documents 3 to 5, there is no countermeasure against the generation of an isolated dot, tone jump, or graininess, so that it may cause serious problems.